1. Not doing the job you were hired to do. “Of course you have higher aspirations, but you have to be willing to schedule appointments and make copies. The day you decide you’re too good for that is the day you’re going to screw yourself. Your boss will stop asking you for help, and you’re going to end up fired because she doesn’t need you.” —Nicole Williams, connection director, LinkedIn

2. Becoming “the oversharer.” “Gossiping is a quick way to lose your credibility, so be careful what you share with people. Your colleagues are not your best friends. I go by the 80/20 rule: 80 percent of your interactions should be professional and productive, and then leave a small chunk for a quick venting session—hey, we’re all human!—or a recap of your weekend.” —Lindsey Pollak, author ofGetting From College to Career

3. Being unprepared. “When given opportunities to prove yourself, there’s no excuse for showing up empty-handed. As a reporter in Chicago, I used to go to diners and ask people what issues they cared about. Yes, it meant working extra, but I sounded more informed when pitching ideas to a news director.” —Nicole Lapin, founder, recessionista.com

And the biggest DON'T of the working world:

4. Letting your mom and dad call the boss (it happens!) “I hear from parents all the time, and it’s not OK. Once I got a call from a father to accept a job on behalf of his son! You should be the one making those decisions, not your parents.” —Jill Valeant, campus recruiter, KPMG